Hydraulic drive circuits for use in hydraulic machines such as hydraulic excavators and cranes each comprise at least one hydraulic pump, at least one hydraulic actuator driven by a hydraulic fluid delivered from the hydraulic pump, and a flow control valve connected between the hydraulic pump and the actuator for controlling a flow rate of the hydraulic fluid supplied to the actuator. It is known that some of those hydraulic drive circuits employ a technique called load sensing control (LS control) for controlling the delivery rate of the hydraulic pump. The load sensing control is to control the delivery rate of the hydraulic pump such that a delivery pressure of the hydraulic pump is held higher by a fixed value than a load pressure of the hydraulic actuator. This causes the delivery rate of the hydraulic pump to be controlled dependent on the load pressure of the hydraulic actuator, and hence permits economic operation.
Meanwhile, the load sensing control is carried out by detecting a differential pressure (LS differential pressure) between the delivery pressure and the load pressure, and controlling the displacement volume of the hydraulic pump, or the position (tilting amount) of a swash plate in the case of a swash plate pump, in response to a deviation between the LS differential pressure and a differential pressure target value. Conventionally, the detection of the differential pressure and the control of the tilting amount of the swash plate have usually been carried out in a hydraulic manner as disclosed in JP, A, 60-11706, for example. This conventional arrangement will briefly be described below.
A pump control system disclosed in JP, A, 60-11706 comprises a control valve having one end subjected to the delivery pressure of a hydraulic pump and the other end subjected to both the maximum load pressure among a plurality of actuators and the urging force of a spring, and a cylinder unit operation of which is controlled by a hydraulic fluid passing through the control valve for regulating the swash plate position of the hydraulic pump. The spring at one end of the control valve is to set a target value of the LS differential pressure. Depending on the deviation occurring between the LS differential pressure and the target value thereof, the control valve is driven and the cylinder unit is operated to regulate the swash plate position, whereby the pump delivery rate is controlled so that the LS differential pressure is held at the target value. The cylinder unit has a spring built therein to apply an urging force in opposite relation to the direction in which the cylinder unit is driven upon inflow of the hydraulic fluid.
However, the above conventional control system for the hydraulic pump has had the following problem.
In the conventional pump control system, the tilting speed of a swash plate of the hydraulic pump is determined dependent on the flow rate of the hydraulic fluid flowing into the cylinder unit, while that flow rate of the hydraulic fluid is determined dependent on both an opening, i.e., a position, of the control valve and setting of the spring in the cylinder unit and, in turn, the position of the control valve is determined by the relationship between the urging force of the LS differential pressure and the spring force for setting the target value. Here, the spring of the control valve and the spring of the cylinder unit each have a fixed spring constant. Accordingly, a control gain for the tilting speed of the swash plate dependent on the deviation between the LS differential pressure and the target value thereof is always constant. The control gain, i.e., the spring constants of the two springs, are set in such a range that change in the pump delivery pressure will not cause hunting and the pump is kept from coming into disablement of control on account of change in the delivery rate upon change in the swash plate position.
In the LS control, the delivery pressure of the hydraulic pump is determined dependent on a difference between the flow rate of the hydraulic fluid flowing into a line, extending from the hydraulic pump to the flow control valve, and the flow rate of the hydraulic fluid flowing out of the line, as well as a line volume into which the delivered hydraulic fluid is allowed to flow. Therefore, when the operation (input) amount of the flow control valve (i.e., the demanded flow rate) is small, the opening of the flow control valve is so reduced that the small line volume between the hydraulic pump and the flow control valve plays a predominant factor. As a result, the delivery pressure is largely varied even with a slight change in the flow rate upon change in the swash plate position. On the other hand, when the operation amount of the flow control valve is increased to enlarge the opening thereof, the large line volume between the pump and an actuator now takes part in the pressure change, whereby change in the delivery pressure upon change in the delivery rate is reduced.
Accordingly, in order to reliably perform the LS control over a range of the entire operation amount (opening) of the flow control valve without causing hunting, the above-mentioned control gain, i.e., the spring constants of the two springs, are set to a relatively small value such that a tilting speed of the swash plate to prevent the pressure change from hunting at the small opening of the flow control valve is provided.
Meanwhile, when a control lever is operated, the operator tends to operate the control lever at a speed corresponding to the speed change demanded for the actuator. With the operating speed of the control lever being small, the difference between the demanded flow rate of the flow control valve and the delivery rate of the hydraulic pump is small, and so is a deviation between a differential pressure signal, determined from the pump delivery pressure and the maximum load pressure, and the target differential pressure set by the spring. In this case, because the operating speed of the control lever is small, the control gain set by the two springs as mentioned above can provide sufficient change in the tilting speed of the swash plate, i.e., the sufficient delivery rate of the hydraulic pump, to realize demanded speed change of the actuator.
However, when the operating speed of the control lever is large, i.e., when the control lever is operated abruptly, there occurs a large difference between the demanded flow rate of the flow control valve and the delivery rate of the hydraulic pump, which also increases the deviation between the differential pressure signal, determined from the pump delivery pressure and the maximum load pressure, and the target differential pressure set by the spring. In this case, with the control gain set by the two springs as mentioned above, the tilting speed of the swash plate, i.e., change in the delivery rate of the hydraulic pump, is limited and becomes insufficient. Accordingly, the demanded speed change of the actuator cannot be realized, causing the operator to feel that the actuator is too slow in action.
To solve the above problem, therefore, the present inventors have proposed in International Application No. PCT/JP90/00962 (International Application Date: Jul. 27, 1990; International Laid-Open No. WO91/02167; International Laid-Open Date: Feb. 21, 1991) a control system for a hydraulic pump characterized in comprising first means for determining, based on a delivery pressure of a hydraulic pump and a maximum load pressure among a plurality of actuators, a target displacement volume (a tilting amount of a swash plate) of the hydraulic pump to reduce a differential pressure deviation between the above differential pressure and a preset target differential pressure, second means for determining a control gain of the first means to becomes larger with the differential pressure deviation increasing and smaller with the differential pressure deviation decreasing, and third means for controlling displacement volume varying means (swash plate) of the hydraulic pump so that the displacement volume of the hydraulic pump is matched with the target displacement volume determined by the first means.
With the above arrangement, in a range where the operating speed of the control lever is small and so is the differential pressure deviation, the control gain determined by the second means also becomes small to reduce the tilting speed of the swash plate. This enables stable control in which there occurs no hunting due to abrupt change in the delivery pressure. On the other hand, when the operating speed of the control lever is large, i.e., when the control lever is operated abruptly and the differential pressure deviation is increased, the control gain determined by the second means also becomes large to raise the tilting speed of the swash plate, thus enabling a response that is not slow but prompt. By so doing, the delivery pressure of the hydraulic pump can always be controlled in an optimum way regardless of the operating speed of the control lever.
The present invention is intended to further improve the above prior application and solve the problem encountered in the case of making the target differential pressure variable.
More specifically, while the target differential pressure between the pump delivery pressure and the maximum load pressure is usually set constant in the load sensing control, it has been proposed to make the target differential pressure variable for various purposes. One example is disclosed in JP, A, 2-76904. In this proposed technique, the target differential pressure can be changed externally for the purpose of facilitating fine speed operation of an actuator. By setting the target differential pressure to a small value, the displacement volume of the hydraulic pump is controlled so as to keep the small target differential pressure. As a result, since the differential pressure across the flow control valve also becomes small by being restricted by the small target differential pressure, metering characteristics of the flow control valve are changed to reduce the flow rate of the hydraulic fluid supplied to the actuator and the fine speed operation of the actuator can easily be realized.
In the case of making the target differential pressure so variable, however, at the small target differential pressure, the differential pressure deviation cannot exceed the target differential pressure and the differential pressure deviation is also limited to a small maximum value, leading to that when the operating speed of the control lever is large, i.e., when the control lever is operated abruptly, there can be obtained only a limited small differential pressure deviation. Accordingly, even if the control gain is set dependent on the differential pressure deviation as with the foregoing prior application, the obtained control gain is small and the tilting speed of the swash plate is so limited that the actuator is forced to move slowly.
An object of the present invention is to provide a control system for a hydraulic pump which, when a target differential pressure for load sensing control is set as a variable value, can perform stable control at a small operating speed of the control means without causing hunting and achieve a response, not slow but prompt, at a large operating speed of the control means, no matter what the value of the target differential pressure.